Satellites and other spacecraft are in widespread use for various purposes including scientific research and communications. These scientific and communications missions, however, cannot be accurately fulfilled without wireless communication between a ground station and the spacecraft. In many applications, the satellite relies upon a wireless communication to send and receive electronic data to perform attitude and position corrections, diagnostic status checks, communication calculations and other functions. Without accurate wireless communication, proper satellite function is hindered and at times adversely effected.
Many modern spacecraft use resonator tuning systems for changing communication frequencies. The prior art systems for changing communication frequencies are predicated upon coupling a voltage controlled capacitor (varactor) to a resonator in order to change its resonance frequency. There are two general cases for resonator tuning systems, the lumped case and the distributed case.
In the lumped case, the resonance frequency .function..sub.0.sub..sub.-- .sub.lumped of a resonator with capacitance C and inductance L is given by: ##EQU1##
Tuning is effected by connecting the varactor having capacitance C.sub.var (V.sub.control), either in series or in shunt with the resonator. When connected in series, the resonant frequency becomes: ##EQU2##
When, on the other hand, the varactor is connected in shunt with the resonator, the resonance frequency is given by: ##EQU3##
In the distributed case, the varactor is used to terminate a transmission line of length equal to one-quarter wavelength at the frequency of interest. Then, by virtue of the impedance inverter effect, the input impedance of the transmission line acts inductive, with an inductance value that is a function of the terminating capacitor value. Thus the effective inductance L, of the resonator being loaded by the transmission line, is changed and consequently its resonance frequency is changed.
The fundamental disadvantage of these approaches stems from the fact that the semiconductor varactor is not a pure capacitor, but contains an intrinsic parasitic resistance that introduces losses in the resonator, thus lowering its unloaded Q. The consequence of a reduction in the unloaded Q may be appreciated by examining the carrier to noise (C/N) ratio in a Voltage Controlled Oscillator (VCO), where (C/N) is given by: ##EQU4##
Where Q.sub.L is the loaded Q of the resonator, Loss is the loss factor in the resonator, f.sub.0 is the frequency of oscillation, .DELTA.f is the offset frequency from f.sub.0, P.sub.0 is the output power of the oscillator, k is Boltzrnann's constant, T is the absolute temperature, B is the measurement bandwidth, and NF is the noise figure of the amplifier. Examination of equation (4) reveals that in order to obtain high C/N ratio, the loaded Q must be high. The loaded Q is highest when it experiences minimum external loading.
The disadvantages associated with these conventional resonator tuning techniques have made it apparent that a new technique for resonator tuning is needed. The new technique should maintain the high, unloaded Q properties of resonators, while significantly reducing phase noise. Additionally, the new technique should allow superior frequency tuning capability. The present invention is directed to these ends.